Phase C: distributed messaging capsule + idle-loop pump

Extract the messaging vocabulary (arenas, MSG-*/CH-*/MBR-* words) out of
capsules/hermes/init.4th into a new shared capsules/common/messaging.4th
that Hermes and Artemis each load at birth, giving every VM its own
private MSG-ARENA/CH-ARENA instead of only Hermes having one. Hermes
stays the owner of the one real, canonical COMMON-CH; Artemis subscribes
into it via VM-EXEC at her own birth, and Hermes proactively subscribes
Hera (idx 0) since Hera always exists first.

Hera does NOT get her own copy: register_child_vm_words()'s own doc
comment explains why the STADIUM-* primitives common:messaging.4th
depends on are deliberately never registered in her dictionary (keeps
her dict_hash off item 4.1's baseline). Confirmed live by loading it
into her dictionary anyway first -- every colon-definition referencing
an unregistered Stadium primitive was silently dropped (MSG-HEAT@/!,
CH-HEAT@/!, MSG-COOL-ALL, MSG-TICK all missing after boot). Reverted
that path; she orchestrates via BIRTH/VM-EXEC/VM-CALL instead.

Added capsule_vm_registry_get_by_index() (capsule_birth.c/.h) for
registry enumeration by birth-order position, and a pump in repl.c's
existing idle hook that walks every live VM once per idle beat and
VM-EXECs MSG-TICK into each one except Hera's own entry.

Verified clean (no UNKNOWN WORD / VM-EXEC errors after birth) on all
three architectures.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019ZGkimpfyh63EZyRkNbkPD

src/starkernel/kernel_main.c

@@ -813,6 +813,20 @@ static void kernel_main_deep(BootInfo *boot_info) {
         }
     }
 
+    /* Phase C (2026-08-28): Hera does NOT get her own common:messaging.4th
+     * arena -- register_child_vm_words()'s own doc comment (mama_forth_
+     * words.c) explains why: the eight STADIUM-* FORTH primitives that
+     * word depends on (CH-HEAT@/!, MSG-HEAT@/! etc. all route through
+     * them) are deliberately never added to register_mama_forth_words(),
+     * to avoid moving Hera's dict_hash off item 4.1's baseline. Confirmed
+     * live: loading messaging.4th into Hera's own dictionary silently
+     * dropped every colon-definition that referenced one of those
+     * primitives (MSG-HEAT@/!, CH-HEAT@/!, MSG-COOL-ALL, MSG-TICK all
+     * missing after boot) -- not a timing bug, a real, pre-existing
+     * constraint. Hera orchestrates via BIRTH/VM-EXEC/VM-CALL directly
+     * instead; the idle-loop pump (repl.c) skips her own registry entry
+     * for the same reason. */
+
     /* Hermes is now a permanent fleet-foundation VM, not self-test
      * scaffolding -- FABRIC-3.md D.7 (birth-by-message-only, 2026-08-28):
      * the Tripod legs (Hera/Hermes/Artemis) must be alive session-less so